This invention relates generally to the processing of measured data. Especially the invention applies to the binning procedure of data which is measured with a CCD (Charge-Coupled Device) sensor unit. The invention is preferably used in photometrics for measuring radiation from samples on a well plate. One purpose of the invention is to achieve improvement in signal-to-noise values of the measurements.
CCD sensors are generally used in photometrics for measuring radiation, such as light, from samples. The samples are usually inserted into wells on a well plate in a photometrics equipment. FIG. 1 shows a prior art arrangement of a photometrics device 100. The samples to be measured are inserted onto a well plate 102. The samples may be excited with radiation from a lamp unit 104. The excitation radiation 106 is reflected by a beam-slitter mirror 108 onto the well plate. The lamp unit 104 is controlled to give radiation with a determined intensity. After a possible excitation, the radiation 110 from the samples is led to a CCD unit 120. A lens system 112 creates an image of the samples onto a CCD screen. The exposure period is controlled with a shutter 116. The radiation is further filtered with an emission filter 114 in order to select the determined wavelength of radiation for the measurement. The lamp unit 104, the shutter 116 and the CCD unit 120 are controlled with a control unit 130. The measurement process is further controlled by a computer unit 140. The computer unit also processes the output data of the measurements to achieve radiation intensity results.
FIG. 2 illustrates registers of a CCD unit 200. The CCD unit comprises a parallel register 210 that consists of a matrix of charge wells 211. As the surface is exposed to radiation, charges are formed into the charge wells according to the intensity of the radiation exposure. A charge pattern thus accumulates in the parallel register. After the exposure the charge wells or xe2x80x9cpixelsxe2x80x9d of the matrix are read by shifting the charges at each row of the parallel register towards a shift register or xe2x80x9cserial registerxe2x80x9d 220. After shifting the charges by one step, the charge wells of the shift register comprise charges of one column of the parallel register. The shift register is then read by shifting the charges at the shift register towards an output charge well or xe2x80x9coutput nodexe2x80x9d 230. After each step of shifting the output node is read. After all the charge wells of the shift register are read, the charges at the parallel register are further shifted by one step. The readout procedure is further repeated until the whole parallel register is read. The measurement data is thus converted into serial set of pixel charge values that present radiation intensities at the pixels. FIG. 2 also shows images of four samples 203 of a well plate. After processing the ouput data, an image can be formed where pixels within the sample image area present the radiation intensity of the corresponding positions within the sample image.
One problem in photometrics is related to the fact that the intensity of the radiation is low and therefore the signal-to-noise ratio of the measurement data may be low. In order to increase the signal-to-noise ratio, binning method is often used. Binning is a technique of combining charge from adjacent pixels during the readout process. The charge is collected as described above, but the readout is programmed differently. With parallel binning, when charge is shifted from the parallel register into the shift register, charge is accumulated from two or more columns before the serial shifting begins. With serial binning, two or more charge packets are similarly accumulated in the output node before the charge is digitalized and read out.
Binning is specified by a binning factor, which is the number of pixels to be combined on the CCD, for example xe2x80x9c2xc3x972 binningxe2x80x9d. If binning is used, the capacity of the shift register and the output charge well must be designed according to the total charge of the pixels that are accumulated in binning.
Binning improves the signal-to-noise ratio and extends the dynamic range of the CCD imager, but at the expense of spatial resolution. Binning is thus useful in applications where resolution is not of primary concern. Because binning reduces the number of pixels to be processed and digitized, the readout speed is also increased. If, for example, 2xc3x972 binning is used, the resolution (number of pixels in the corresponding direction of the image) becomes half of the corresponding resolution without binning, and the signal-to-noise value becomes twice as good as the corresponding value without binning. This improvement of signal-to-noise value is related to the noise from the parallel register of the CCD unit. However, the signal-to-noise value related to the readout noise improves even by the binning factor. Binning has been advantageous in photometrics, because there has been no need for obtaining a high resolution.
There has been a requirement of increasing the measuring capacity by increasing the number of samples on a well plate. Therefore, instead of well plates with 8xc3x9712 wells there are now well plates with 32xc3x9748=1536 wells, for example. The size of wells have accordingly become smaller. FIG. 3 shows a well plate 302 with 32xc3x9748 wells 303 in an actual size. FIG. 4 further shows an image of 4 samples 403 on the surface of a CCD unit 400. Especially when the well plates with small wells are used there is a following disadvantage with the prior art binning method: When the distance between the sample wells is small, the number of CCD charge wells 411 within the area between the samples is also small. In the example of FIG. 4 there is only 3 pixels between the sample images. If the binning factor is close to the number of pixels in that area, it is possible that radiation from a sample well affects the result of the neighbouring pixel well thus causing interference between sample images. Therefore it is not possible to adequately separate the radiation of the neighbouring sample wells. Another problem is that it is not possible to achieve an adequate signal-to-noise ratio from the radiation of the small samples in order to get accurate results in a moderate imaging time.
It is the objective of the present invention to create a solution, in which a good signal-to-noise value is achieved, and interference between the measuring results from different positions can still be avoided. This objective is achieved in photometrics application by selecting the binning factor on the basis of position and size of sample wells on a well plate.
The number of pixels that are accumulated preferably equals the number of pixels in within the sample well image. Thus, if there are e.g. 15 pixels within the sample image row or column, they are preferably all accumulated in binning. The pixels that are outside the sample image area can be ignored. For example, in the arrangement of FIG. 4 the pixels xe2x80x9cAxe2x80x9d would be parallel binned and the pixels xe2x80x9cBxe2x80x9d would be serial binned in the readout process. The rest of the pixels could be ignored.
With the present invention it is possible to achieve a maximal signal-to-noise ratio, because all pixels within the image area of one sample are accumulated. However, since the pixels between the sample images are not accumulated with the pixels within the sample images, the problem of interference between samples can be minimized.
The invention applies to a method for measuring radiation from an object with a charge coupled device comprising a matrix of pixels arranged in rows and columns, in which method
the radiation creates charges to the charge wells of the pixels,
charges from a column of the pixels is shifted to a shift register,
the charges in a shift register are shifted to an output charge well,
the charge is measured from the output charge well and
charges from at least two pixels are accumulated into the output charge well, characterised in that the pixels whose charges are accumulated are determined on the basis of the position and the dimension of a detail in the object.
The invention also applies to an arrangement for measuring radiation from an object comprising a charge coupled device with a matrix of charge wells arranged in rows and columns of pixels, the arrangement comprising
a shift register for receiving charges from a column of the parallel register pixels,
output well for receiving charges from the shift register,
means for measuring the charge from the output well and
means for accumulating charges from at least two pixels characterised in that the arrangement further comprises means for determining the accumulated pixels on the basis of the position and the dimesion of a detail in the object.